There exist a number of fluid handling devices employing a pair of cooperating screw rotors. Generally, these devices include a casing with a pair of operating chambers defined by two parallel bores (e.g. cylindrical bores). A male rotor and female rotor are disposed in the parallel bores, and cooperate together during operation. For example, in a compressor one bore provides a common intake port and the other bore provides a high pressure discharge port. Typically, the male and female rotors have a wrapping angle of less than 360.degree..
The greater part, if not all, of the lands and troughs on the male rotor lie outside the pitch circle, while the greater part, if not all, of the troughs and lands on the female rotor which engage with the aforesaid male rotor lie within the pitch circle. Generally, the male rotor will have four (4) lands, and the female rotor will have six (6) lands.
A "land" for purposes of the present invention is defined as the protruding portion of each tooth, and located between adjacent troughs. A "trough" for purposes of the present invention is defined by the concave portion located between adjacent lands.
In related screw rotor fluid handling devices, the set of rotors are driven synchronously by means of synchronized gears. In some devices, the two rotors are driven in such a way that they do not come in contact with each other (i.e. non-contact type). In other devices, one of the rotors (i.e. the male rotor) serves as a drive rotor and contacts with the other rotor (i.e.female rotor) imparting rotary torque thereto so that both rotors are rotated together.
However, in the related non-contact type fluid handling devices, the synchronized gears must operate with great precision in order to avoid direct contact between the rotors driving up the cost of manufacture.
For this reason, the majority of screw-type fluid handling devices currently in use employ a rotary scheme by which the rotors come in direct contact with each other. The tips of the lands on the female rotor extend beyond the pitch circle, forming addendum. The troughs located between adjacent teeth on the male rotor that engage with the addendum lie within the pitch circle, forming dedendum. This arrangement scheme has replaced most previous designs. The term "addendum" for purposes of the present invention refers to the tips of the lands, which extend beyond the pitch circle, and the term "dedendum" refers to the bases of the troughs between adjacent teeth located within the pitch circle.
This type of rotor arrangement is widely used in oil jet type rotor devices, however, its use is not limited to this type of application. It can also be used in oil-less type rotor devices.
These related fluid handling device encounter some problems during operation. For example, referring to the related device shown in FIG. 4, an addendum 21 is provided on a female rotor 2. This is a contact type compressor employing screw rotors of a type as disclosed in Japanese Patent Publication 56-17559. As the male and female rotors rotate together, the addendum 21 on female rotor 2 engages with and disengages from the base 11 of trough 13 of the male rotor 1. As the screw rotors rotate together, a pocket 4 initially forms between the surfaces of the teeth of both rotors and by plate 3, and then decreases in volume size as the rotors further rotate while an escape path 41 communicating with an escape chamber of pocket 4 becomes more narrow. This situation causes exit resistance in the operating fluid leaving pocket 4 resulting in the exit becoming semi-occluded. Eventually as the escape path 41 is closed down, the fluid is compressed in the pocket 4, and work required for compression of the trap fluid in the pocket 4 is wasted.
If it should happen that the fluid trapped in the pocket 4 contains an impurity such as oil from an oil jet mechanism, or operating fluid condenses within the pocket 4, not to mention the various trapped gases located in the pocket 4, significant vibration and noise can be generated when the fluid is compressed. Furthermore, as the work required for compression of trapped fluid is increased, the efficiency and reliability of the compressor will decrease substantially.
In Japanese Patent Publication 2-50319, a design is suggested whereby the addendum 21 on the female rotor 2 is provided with a curvature matching the profile of the base dedendum on the male rotor 1. However, with this rotor arrangement, a semi-occluded pocket 4 can still form as can be seen in FIG. 5, even though it is much smaller than the pocket 4 of the arrangement shown in FIG. 4. The perfect solution to one problem results in this unrelated problem in the arrangement shown in FIG. 5.
Another problem with existing related devices concerns the possibility of forming a blowhole. This situation can occur when a screw rotor device is constructed with a female rotor 2 having addendum 21 located beyond the pitch circle. Along the line of the seal between the tips of the lands on the male and female rotors and the cylindrical wall of the operating chamber, the apices of the V-shaped chambers coincide with corresponding points along the associated line on the bore of the corresponding operating chamber. Thus, different V-shaped chambers are completely sealed with respect to each other, and theoretically there are no blowholes.
However, when addendum 22 are provided on the aforementioned female rotor 2, as shown in FIG. 6, the points at which the cylindrical bores intersect cannot extend as far as the aforementioned pitch circle. Thus, a triangular ventilation hole known as a "blowhole" will be formed by one edge of point 5 of the intersection of the bores, the top of land 12 on male rotor 1, and the advancing flank of addendum 22 on female rotor 2. The term "flank" for purposes of the present invention refers to the side of either an advancing or retreating land.
To address this problem, Japanese Patent Publication 3-4757 proposes making the troughs on the female rotor 2 arcs, generated curves, or hyperbolae, while the curves of the advancing flanks which start at the bases of the troughs between teeth and end at addendum 22 would be unique curves, not arcs, whose radii would vary with the angle of the profiles. The lands on the male rotor 1 would be arcs or generated curves; the curves of the retreating flanks on the tops of the aforesaid lands would be unique curves, not arcs, whose radii would vary with the angle of the profiles. This would minimize the area of the aforementioned blowholes.
Generally, in screw rotor devices the length of the seal line varies inversely with the area of the blowholes. When the blowholes are minimized by matching the troughs on the female rotor 2 with the lands 12 on the male rotor 1 as in the related devices, it becomes extremely difficult to shorten the sealing line.
In Example 1 discussed above, the problem is addressed by having the angle .gamma. of the tangent to the retreating flank of the trough on the female rotor 2 approach 90.degree.. However, as can be seen in FIG. 3, this does not sufficiently shorten the sealing line.